THE YEAR IN IDEAS: A TO Z.; The Game That Plays You

Last spring, an unusual game appeared on the Web -- though ''game'' isn't quite the right word. It was more like an original and expansive fictional world, mapped out on more than 1,000 interwoven Web pages. The untitled, unannounced creation was intended as a promotion for Steven Spielberg's film ''A.I.: Artificial Intelligence,'' and though its success as a promotional tool is debatable -- its very existence was a secret, even from many who were working on it -- as an interactive game, it broke new ground.

The ''A.I.'' game was the brainchild of Jordan Weisman, the creative director of Microsoft Games. Weisman had long been fascinated with the folklore that arose around Paul McCartney's rumored death, which kept fans poring over album covers and song lyrics for clues. Weisman envisioned a game that would invade the real world of its players with an equally relentless fiction. ''What Jordan wanted,'' says Elan Lee, one of Weisman's early collaborators, ''was for you to be sitting in your boardroom in the middle of a big meeting, and your cellphone would ring, and it would be the game, calling you.''

On its face, the game was a murder mystery. It invited Web surfers to help a young woman solve the murder of Evan Chan, who may or may not have been killed by an errant robot. Players were first teased into the fictional world by a provocative credit in the ''A.I.'' trailer: ''Jeanine Salla -- Sentient Machine Therapist.'' A Google search for Salla's name led to the fictional character's home page. From there, players stumbled, link by link, through an astonishingly large, diverse and odd collection of Web sites: anti-robot militia pages, pro-A.I. pages and phony corporate sites offering intelligent robotic homes. The pages were sprinkled with phone numbers that led to dummy voice-mail boxes and e-mail addresses that begged to be written to (and that responded when you did). It was a puzzle whose precise size, shape and purpose was impossible to measure, and because it did not announce itself as a product, or ask for your credit card, players were tempted to think of the game as their own unique discovery.

The game was designed to be too complicated for a single player to solve; its puzzles required intimate knowledge on subjects as diverse as the enigma code and 16th-century lute tablature. So players began to locate one another and exchange information, and eventually 7,000 curious souls created a Web site of their own to crack the game's increasingly arcane riddles.

Similarly interactive Web games were developed for the films ''Swordfish'' and ''Planet of the Apes,'' but to a much more muted effect. And in August, Electronic Arts introduced Majestic, a reality-invading game that envelops its players in a conspiracy of e-mail messages, faxes, phone calls and instant messages. But Majestic's $10-a-month subscription presents both a financial and an imaginative barrier to entry; reality isn't as much fun when you know it's just a game. John Hodgman